In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist, application and heating of the photoresist are significant factors in achieving desired critical dimensions. The photoresist should be heated such that a uniform heating rate is maintained in order to insure uniformity and quality of the photoresist layer. The photoresist layer thickness typically is in the range of 0.1 to 3.0 microns. Good resist thickness control is highly desired, and typically variances in thickness should be less than .+-.10-20 .ANG. across the wafer. Very slight variations in the photoresist thickness may greatly affect the end result after the photoresist is exposed by radiation and the exposed portions removed.
One of the variables in heating of photoresist film is the temperature at which heating occurs. In order to achieve uniform heating, a uniform temperature in a curing or enclosure chamber is necessary. Small changes in the time/temperature history of the photoresist can substantially alter image sizes, resulting in lack of image line control. A uniform time/temperature history of the photoresist is especially important with chemically amplified photoresists because image size control may be drastically affected by only a few degrees difference in temperature. Often substantial line size deviations occur when the temperature is not maintained within 0.5 degree tolerance across a silicon wafer. For example, when a photoresist is baked onto a substrate (e.g., wafer), temperature tolerances of .+-.0.2.degree. C. are required.
An efficient system/method to uniformly heat a layer of temperature-sensitive film formed on a semiconductor substrate is therefore desired to increase fidelity in image transfer.